1. Field of the Invention
This invention relates to photosensitive materials for use in electrophotography wherein organic photoconductors are used for the formation of images for electrophotography and also to a method for making the photosensitive materials or mediums.
2. Description of the Prior Art
As a photoconductor used to constitute a photoconductive layer in photosensitive materials for use in electrophotography, there have been widely used inorganic materials or elements such as Se, CdS, amorphous silicon and the like. These photoconductors have, respectively, inherent drawbacks and are usually used in combination so as to make up the drawbacks of the respective photoconductors. However, existing photosensitive materials using inorganic photoconductors are not necessarily satisfactory in practical applications.
For instance, with Se, it is the usual practice to add Te or As in order to enlarge the range of spectral sensitivity with an attendant disadvantage that the light fatigue increases. Se, Te and As are all harmful to the human body. Although the Se photoconductor layer is usually applied in the amorphous state, this amorphous state will be lost at a temperature of about 65.degree. C. or over by recrystallization, which leads to the deficiency that the dark resistance decreases.
When using CdS or ZnO, it is necessary to disperse it in appropriate resins, which presents problems with respect to the reproducibility of photoconductive characteristics and moistureproofness of the photoconductive layer. CdS is also harmful to the human body and ZnO has low photosensitivity.
While amorphous silicon has good photoconductive characteristics and durability, the manufacture process is complicated and thus, the amorphous silicon is expensive.
In place of these inorganic photoconductors, organic photoconductors which are often abbreviated to OPC have been recently studied and developed. Several organic photoconductors have now been in use. There is the recent trend toward the use of semiconductor laser devices as light sources. This promotes the developments of organic photoconductors which have sensitivity in a near infrared region. In general, organic photoconductor materials have not only the merit of ease in preparation, but also the advantages such as the capability of relatively easily varying a range of wavelength for photosensitivity by changing a molecular design with respect to the visible light sensitivity, good film-forming properties and lightweight properties.
The photosensitive materials for electrophotography using organic photoconductor materials are those of the builtup type which include a charge-generating layer (which may be hereinafter referred to simply as CGL) wherein carriers are generated by absorption of light, and a charge transfer layer (which may be hereinafter referred to simply as CTL) wherein the generated carriers are transferred. The respective photoconductive layers are formed by applying a charge-generating material and a charge transfer material, after mixing with binder resin, if necessary, onto a substrate such as a drum or a belt. It will be noted that in the photosensitive material of the builtup type, whether the CGL or CTL is formed as an upper layer depends on whether the charging is positive or negative.
Thus, the organic photoconductor material has the advantage that the photoconductive layer or layers can be formed by a simple coating process, but further improvements have been demanded with respect to the chemical stability or durability relative to environmental changes as well as sensitivity. More particularly, aside from the photosensitivity, the chemical stability against ozone or NO generated during the charging process, light fastness and wear resistance have to be further improved.